Allow incremental marking when expose_gc is turned on.

src/extensions/gc-extension.cc

Index: src/extensions/gc-extension.cc
diff --git a/src/extensions/gc-extension.cc b/src/extensions/gc-extension.cc
index 813b9219bfec7eacd0638cf002c8abb368bba3d2..2282075445adba3950ae5c53599926469886c20a 100644
--- a/src/extensions/gc-extension.cc
+++ b/src/extensions/gc-extension.cc
@@ -43,7 +43,7 @@ v8::Handle<v8::Value> GCExtension::GC(const v8::Arguments& args) {
   if (args[0]->BooleanValue()) {
     HEAP->CollectGarbage(NEW_SPACE, "gc extension");
   } else {
-    HEAP->CollectAllGarbage(Heap::kNoGCFlags, "gc extension");
+    HEAP->CollectAllGarbage(Heap::kAbortIncrementalMarkingMask, "gc extension");
   }
   return v8::Undefined();
 }